Reference may be made to the following U.S. patents of interest: U.S. Pat. Nos. 3,581,094; 3,643,095; 3,764,808; 3,863,073; 3,875,411; 3,936,647; 3,947,689.
In presently available X-ray apparatus, there is provided an image receptor and an adjustable shutter mounted between the X-ray source and the image receptor. In accordance with prescribed health and safety regulations, automatic or semi-automatic means are provided for adjusting the shutter aperture to conform to the image receptor size and the detected distance between the X-ray source and the image receptor. The detected distances may be either a predetermined discrete distance (known as discrete source to image distance or discrete SID) or a continuously monitored distance (known as continuous source to image distance, or continuous SID). Such shutter adjustment operations are required in X-ray apparatus in order to insure that a person being X-rayed will only be exposed to a minimum of useful X-rays. The term "Positive Beam Limitation" or PBL is commonly used in the trade to identify this desired apparatus and/or operation. Thus, in such prior art PBL X-ray apparatus, the shutter aperture is automatically adjusted (and special lighting features may be provided in manual operation) so that the X-ray will only fall on the actual cross-sectional area of an image receptor or X-ray film.
In such apparatus, the dimensions of two rectilinear axes, commonly termed the "longitudinal" and the "cross" axes of the image receptor cassette are normally aligned with the corresponding longitudinal and cross axes of the shutter aperture or collimator. Thus, the shutter aperture along the longitudinal axis as well as the shutter aperture along the cross axis is adjusted in accordance with the detected source to image distance and the detected image receptor size so that the area covered by the X-ray beam is only impinging on the image receptor.
In such prior PBL X-ray apparatus, it has become standard procedure to allow the operator to remove the image receptor cassette from its holder during the X-ray operation for various reasons, such as for patient comfort during X-ray filming, or to enable a more effecient filming procedure. During such a procedure, the operator may then normally swivel or rotate the collimator in a plane parallel to the plane of the image receptor, to align the collimator longitudinal and cross axes with those of the image receptor. It has now been found that such an operation can lead to unwanted exposure of X-rays. This may occur if, for instance, following this procedure the operator does not return the collimator back to the home or non-swivel position. Thus, when the next operator inserts a cassette into the holder, with the collimator in the swivelled position, the longitudinal and cross axes of the collimator and image receptor are now misaligned. Upon activating the X-ray source, the X-rays will undesirably impinge on an area of the patient's body which is not being filmed.
Accordingly, it is desired to prevent the operation of the X-ray source if the collimator is in the swivelled position and a cassette is in the holder. In addition, in certain situations it is desired to enable the collimator to be swivelled 90.degree.. In such conditions, it is desired to detect such 90.degree. swivelling and provide for the exchanging of the longitudinal and cross axes information with regard to the image receptor so that the collimator and the image receptor axes generally conform to each other as in the normal home position. Furthermore, it is desired to prevent premature collimator shutter adjustments in such PBL X-ray apparatus.